The objective of the proposed research is to increase our understanding of the synaptic mechanisms used by local circuit neurons and, in particular, to study the function of graded synaptic transmission (GST) in spiking, local circuit neurons. The immediate objective is to understand the functioning of GST in an invertebrate model system, the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. This study of local circuit neurons will describe neuron-to-neuron GST behavior and compare it to the behavior of spike-evoked postsynaptic potentials (PSPs) in the same neurons. A detailed study of the mechanisms underlying GST and a comparison to those of PSPs will focus on voltage dependent ionic currents (e.g., presynaptic calcium current). Attention will be given to the plastic properties of synapses, including time dependent effects. Methods to be used in this study include intracellular somatic and neuropil recording and whole cell voltage clamp techniques. Little is known about GST and less about GST as it occurs in spiking neurons. GST and PSP properties and their underlying mechanisms have never previously been examined in neurons which use both GST and PSPs as a part of normal function. However, many cells involved in local neuronal circuits in the major invertebrate phyla and in mammals (including man) probably use both GST and PSPs as methods of synaptic communication. This research should be a first step toward understanding how these neurons function.